5'-FMN is a compound which plays a quite vital role as a coenzyme for various enzyme reactions in the living body, and is used as an additive of pharmaceuticals, foods, and feeding stuffs. Further, 5'-FMN is widely used as a starting material for flavine-adenine dinucleotide (hereinafter referred to as FAD) which is used as a therapeutic agent against vitamin B.sub.2 deficiency.
Industrially, 5'-FMN is usually produced by reacting riboflavine with a phosphorylating agent such phosphoryl chloride. Accordingly, 5'-FMN produced contains impurities such as unreacted riboflavine, and 4'-FMN, riboflavine-4'-5'-cyclophosphate and riboflavine-phosphosphoric acid formed as by-products.
Since these impurities have analogous structures to that of 5'-FMN, it is difficult to separate the latter from the former. Especially, 4'-FMN has a very analogous structure to that of 5'-FMN, and therefore, it is extremely difficult to separate the latter from the former.